BAGHDAD — Bombs killed nearly 60 people in Iraq on Thursday in the worst violence since U.S. combat troops withdrew from urban areas last week, and American forces released five Iranian officials suspected of aiding Shiite insurgents.

U.S. officials said they believe the Iranians, detained in northern Iraq in January 2007, had facilitated attacks on American-led forces but handed them over to the Iraqi government at its request because they were obliged to do so under a U.S.-Iraqi security agreement.

The U.S. State Department said it was concerned their release could present a security threat to American troops in Iraq.

Iraq’s foreign minister, Hoshyar Zebari, called the release a “good initiative” that could encourage dialogue between Washington and Tehran, which are longtime foes.

Iranian Embassy spokesman Amir Arshadi said Iraq had transferred the Iranians, described by their government as diplomats, to the embassy. Washington believes they are associated with the Quds Force, part of Iran’s Revolutionary Guard Corps, and that they trained Iraqi militants.

The carnage within Iraqi borders Thursday was a sign that insurgents remain intent on destabilizing Iraq as the United States shifts its focus to the war in Afghanistan.

Attacks are down sharply from past years of war and militants have been driven from many strongholds, but they routinely inflict casualties in Baghdad and northern Iraq, a cauldron of ethnic and sectarian tension.

The day’s violence began at 6:30 a.m., when a suicide bomber in a police uniform knocked on the door of an investigator in the anti-terrorism police force in Tall Afar. When the officer opened the door, the bomber detonated his explosives belt, killing the officer, his wife and son, said Maj. Gen. Khalid al-Hamadani, police chief of the northern Nineveh province.

As people gathered in the aftermath, another suicide bomber detonated his explosives belt, al-Hamadani said. The coordinated attack killed a total of 38 people and injured 66. Army Brig. Abdul-Rahman Abu Raghef said the first suicide bomber was a local resident who had been jailed for one year on suspicion of terrorism but was released in an amnesty in June.

Insurgents also struck Baghdad on Thursday morning, detonating bombs that killed 18 people and injured dozens. Eight of them died and 30 were injured in coordinated blasts near an outdoor market in the Shiite district of Sadr City, said Maj. Gen. Qassim al-Mousawi, spokesman for the city’s operations command center.

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